A self-cleaning type kneading extruder is known in which the surfaces of multiple intermeshing screws are cleaned by contact with each other or with screw flights. Extruders of this type have the function of removing the fluctuations in output, or the rate of stock extrusion, which are caused by blocking or powders in a solid-conveying zone and/or plasticizing zone, and of preventing the degradation of the extrudate caused by the sticking or flowing out of the degeneration products in a melt-conveying zone, and have recently attained importance for use in plasticizing, melting and kneading of polymeric materials and their compositions.
Generally, a "self-cleaning type" extruder has a specified configuration defined by the contour of screws in a cross section at right angles to their axes. It includes at least two screws adapted to rotate in the same direction each of which has one or multiple screw flights, and which are in such a relation that at any position in at least a part of the screws which lies in the longitudinal direction of the extruder, the contour of one screw in a cylinder cross-section taken at right angles to the screw axis is in substantial contact at one point with the contour in the same cross-section of another screw intermeshing therewith.
FIG. 1(A) of the attached drawings is a top plan view of the screw section of a typical known self-cleaning type twin-screw extruder in which each screw S.sub.1 or S.sub.2 has three flights F.sub.1, F.sub.2 and F.sub.3. The cross-section of these screws S.sub.1 and S.sub.2 at a certain point in the longitudinal direction, for example, the cross-section taken along the line I--I of FIG. 1(A), is as shown in FIG. 1(B). It is appreciated from this drawing that the contour of the cross section of screw S.sub.1 is in contact with that of screw S.sub.2 at one point only.
The tops T.sub.1, T.sub.2 and T.sub.3 of screw flights F.sub.1, F.sub.2 and F.sub.3 for each screw S.sub.1 or S.sub.2 in the known self-cleaning type extruders make substantial contact with the inner wall surface of cylinder C, and each screw channel A.sub.1, A.sub.2, . . . A.sub.12 or B.sub.1, B.sub.2, . . . B.sub.9 forms a space substantially completely isolated from the neighboring channels. Accordingly, the stock fed into channel A.sub.1 of screw S.sub.2 necessarily moves to channel B.sub.1 of screw S.sub.1 during the rotation of the screws. Then, it moves to channel A.sub.6 of screw S.sub.2, further passes channel B.sub.6 of screw S.sub.1, and channel A.sub.11 of screw S.sub.2. The stock fed into channel A.sub.2 of screw S.sub.2 is likewise conveyed through the channels A.sub.2 .fwdarw.B.sub.2 .fwdarw.A.sub.7 .fwdarw.B.sub.7 .fwdarw.A.sub.12.
Thus, in the conventional self-cleaning type extruder, the flow path of the stock is completely dependent on the screw channel into which the stock has initially been fed. As described above, the stock which has flowed into the channel A.sub.1 of screw S.sub.2 follows figure 8-patterned helical path A.sub.1 .fwdarw.B.sub.1 .fwdarw.A.sub.6 .fwdarw.B.sub.6 .fwdarw.A.sub.11. Hence, the resin which has flowed into channel A.sub.1, either partly or wholly, never gets together with the stock within the channels A.sub.2, A.sub.3, A.sub.4, A.sub.5, B.sub.2, B.sub.3, B.sub.4, and B.sub.5. In other words, no exchange of stock takes place between channels of one screw and between channels of two intermeshing screws.
Since there is no exchange of the stock between channels in the axial direction of screws in the known selfcleaning type kneading extruder, it has the defect of having insufficient ability to knead and or disperse the resin fed into it.
In an attempt to remedy this defect, it has been suggested in the past to provide the extruder with, besides the feeding screws, kneading discs which are triangular and force side-to-side transfer while kneading and shearing the resin (see SPE Journal, September 1969, pages 1147-1154). However, in this suggested extruder, exchange of the resin between channels is performed only through a limited length at a fixed position in the axial direction of the screws. Consequently, the resin undergoes an abrupt physical action within a short period of time, which frequently produces undesirable results. These discs of the suggested extruder do not have the power of conveying the stock forward, and therefore, the overall conveying power is reduced.
It would be worthwhile here to consider the shearing action of the screw flights in the above conventional self-cleaning type extruder. Generally, the magnitude of the shearing force of a screw independent of the properties of the stock and the operating conditions is given by the product of the average shear rate (velocity gradient) and the time. Since the time depends upon the various dimensions of the screws in the axial direction, the shear rate at the wall surface will be a representative parameter of the shearing action which is associated with the shape of the extruder in its cross section at right angles to its axis.
The shear rate (r.sub..omega.) at the wall surface is generally proportional to R/H [r.sub..omega. .alpha.R/H]. R represents the outside diameter of each screw, and H represents the depth of a screw channel. If the distance between the axial cores of two intermeshing screws is L, then the following equation can be derived because H=R-L. ##EQU1## As is clearly seen from equation (1), the shear rate at the wall surface increases when the distance L between the axial cores of the screws approaches the outside diameter (R) of the screw flight and thus the depth (H) of the screw channel decreases. On the other hand, with decreasing depth (H) of the screw channel, the volume of the intermeshing part of the screws decreases, and consequently, the time during which the stock undergoes a shearing action decreases. This leads to the reduction of the resin kneading action and a drastic decrease of the output.
The screw flights of the multiple screws of the aforesaid kown self-cleaning type extruder have the same outside diameter (R). If R and L are constant, then the depth H (=R-L) of a channel is determined unequivocally, and therefore, the wall-surface shear rate is also determined unequivocally. This means that in order to obtain a high shear at a given rotating speed of screws in the known self-cleaning type extruder, it is necessary to decrease the outside diameter R of the screw flight as compared with the distance L between the axial cores of two intermeshing screws, namely to decrease the depth of the screw channel. If this is done, however, the resin kneading action of the screws will be reduced, and the rate of extrusion of the resin will be drastically decreased, as stated hereinabove. In other words, the conventional self-cleaning type extruders have the serious defect that in order to attain a high shear rate at the wall surface, the kneading action, the resin conveying efficiency, and the rate of extrusion or output must be sacrificed.